Permeable pavement (PP) is a modified form of asphalt
or concrete with a top layer that is pervious to water
due to voids intentionally created during mixing. PPs
include pervious concrete, porous asphalt, and interlocking
concrete pavers. These materials are used as
stormwater treatment practices in urban areas. They
are used in place of traditionally impervious surfaces
to allow infiltration and storage, thus reducing runoff
(see figure 1).

Infiltration practices provide temporary surface and/or
subsurface storage, allowing infiltration of runoff into
soils. In practice, an excavated trench is usually filled
with gravel or stone media, where runoff is stored in
pore spaces or voids between the stones (see figure
1). These systems can reduce significant quantities of
stormwater by enhancing infiltration, as well as provide
filtering and adsorption of pollutants within the
stone media and soils. Infiltration practices are part of
a group of stormwater treatment practices, also known
as best management practices (BMPs)

A stormwater filtering practice (FP) treats stormwater
runoff by passing it through an engineered filter media
consisting of either sand, gravel, organic matter, and/
or a proprietary manufactured product, collecting it in
an underdrain, and then discharging the effluent to a
stormwater conveyance system. FPs are stormwater
treatment practices that are often obtained from the
marketplace due to unique proprietary technologies
(see figure 1).

Wet ponds (WP) are ponds or lakes which provide treatment
and storage of stormwater. The water depth is set
by a structure known as an outlet structure. Wet ponds
are probably the most well-known best management
practice for treatment of stormwater. Because of their
size, they are usually designed to include storage above
the normal pool elevation. This added storage can provide
reductions in downstream flooding and assist in
protecting stream channels. They tend to be large; in
some cases, they can become a passive community
amenity (See Figure 1).

Extended detention ponds (EDs) are dry detention ponds
that provide 12 to 24 hours of runoff storage during peak
runoff events (see figure 1). Releases from the ED ponds
are controlled by an outlet structure. During a storm
event, as the discharge restriction is reached, water backs
up into the ED pond. The pool slows flow velocities and
enables particulate pollutants to settle. Peak flows are
also reduced. ED ponds have the lowest overall pollutant-
removal rate of any stormwater treatment option,
so they are often combined with other upstream, lowimpact
development (LID) practices to better maximize
pollutant-removal rates. Due to their placement at the exit
point of the watershed, ED is often the last opportunity
to treat stormwater before it is discharged to a stream.
Because of its low treatment performance, an ED should
be viewed as the treatment option of last resort.

In the Commonwealth of Virginia, water resources are increasingly being scrutinized due to changing surface water or groundwater availability. Access to good quality water is a continuing concern, and in many communities, managing water use — particularly consumptive use — is a priority to conserve public water supplies to meet the needs of a growing population.